The electromagnetic spectrum encompasses radiation from gamma rays, x-rays, ultra violet, a thin region of visible light, infrared, terahertz waves, microwaves, and radio waves, which are all related and differentiated in the length of their wave (wavelength). All objects, as a function of their temperatures, emit a certain amount of radiation. For example, the higher an object's temperature, the more infrared radiation is emitted.
Thermal cameras can detect this radiation in a way similar to the way a photographic camera detects visible light and captures it in a photograph. Because thermal cameras detect and capture infrared light, thermal cameras can work in complete darkness, as ambient light levels do not matter and are not needed. Color thermal cameras require a more complex construction to differentiate wavelength and color has less meaning outside of the normal visible spectrum because the differing wavelengths do not map uniformly into the system of color visible to and used by humans.
The monochromatic images from infrared cameras are often displayed in pseudo-color, where changes in color are used, as opposed to changes in intensity, to display changes in the signal, for example, gradients of temperature. This is useful because although humans have much greater dynamic range in intensity detection than color overall, the ability to see fine intensity differences in bright areas is fairly limited. Therefore, for use in temperature measurement, the brightest (warmest) parts of the image may be colored white, intermediate temperatures reds and yellows, transitioning to blues and greens, with the dimmest (coolest) parts black. Typically a scale may be shown next to a false color image to relate colors to temperatures.
Thermal cameras have many applications, particularly when light and visibility are low.
For example, thermal cameras have been used in military applications to locate human beings or other warm entities. Warm-blooded animals can also be monitored using thermographic imaging, especially nocturnal animals. Firefighters use thermal imaging to see through smoke, find people, and localize hotspots of fires. With thermal imaging, power line maintenance technicians locate overheating joints and parts, a telltale sign of their failure, to eliminate potential hazards. Where thermal insulation becomes faulty, building construction technicians can see heat leaks to improve the efficiencies of cooling or heating air-conditioning. Thermal imaging cameras are also installed in some luxury cars to aid the driver at night. Cooled infrared cameras can be found at major astronomy research telescopes, even those that are not infrared telescopes.